The growing concerns over developing economic and environment friendly coatings, adhesives and printing inks has led attention towards photo-polymerization of aqueous photo-curable systems. Current methods of coating (wood/automotive) result in considerable emission of volatile organic compounds that are harmful to humans and the atmosphere. To prevent or minimize such an emission, new developments in the field focus on formulations with a low solvent content, water based paints, and IR- or UV-curing systems. Among these, developing water borne UV curing systems offers several economical productivity advantages in terms of production time (fast curing/production efficiency), energy consumed (low-temperature processing; no high energy-consuming drying ovens), stability (higher shelf life, no pot-life issues), safety (little or no use of reactive diluents, non-toxic and non-flammable water base), formulation (ease of changing viscosity, color and gloss; ease of equipment clean-up) and quality of final product (scratch and chemical resistance).
Interestingly for processing of hydrogels using 3D printing/stereolithograpy (SLA), an aqueous photo-curable system is a prerequisite. The most promising application of the three-dimensional (3D) printing in soft tissue engineering is the fabrication of hydrogel-based scaffolds with predesigned structures. Complex 3D hydrogel scaffolds with a fully interconnected structure with predefined dimensions and porosity are required for effective repair or regeneration of tissues and organs. Hydrogel based scaffolds are of specific interest to tissue engineering because they provide high water content environment enabling high-cell encapsulation densities. Among the various rapid-prototyping techniques available for 3D printing of hydrogel 3D structures, stereolithograpy (SLA) can enable fabrication of patient specific hydrogels with high speed, high resolution and computer-aided design capabilities.
In general, processing of hydrogels using SLA based 3D printing involves aqueous solution of an oligomer or reactive monomers, photo-initiator (PI) and/or cross-linking agent. The photoinitiator plays a crucial role in determining the rate of polymerization and consequently the resulting properties of the printed object and the time required for the 3D fabrication. Ideally for fabricating hydrogels with live cells mild processing conditions are desirable, avoiding heating, stirring, use of organic solvents or UV exposure. However, in the absence of highly efficient water-soluble photoinitiators, the majority of current protocols for fabricating hydrogels utilize non efficient, poorly-water soluble photoinitiators which require substantial agitation and/or heating or mixing with organic solvents to obtain clear precursor solutions.